Temperature sensors are needed in a large number of disparate situations. Of particular interest here are resistance temperature detectors, commonly referred to as "RTD's". RTD's are among the most widely used temperature sensors for electrical temperature measurement applications. They are generally reasonable in size and production cost, and are useful over a wide range of operating temperatures. For example, platinum RTD's are used as interpolation standards from the oxygen point (i.e., -182.96.degree. C.) to the antimony point (i.e., 630.74.degree. C.) As it is a a passive, resistive element, an RTD requires signal conditioning circuitry in order to generate a voltage or current signal indicative of its resistance value and, therefore, the RTD temperature. While many such signal conditioning circuits exist, these prior art conditioning circuits generally are unsuited to monolithic fabrication with an RTD in a single integrated circuit. Additionally, prior art signal conditioning circuits do not function well in automotive applications, an environment for which the present invention is particularly intended.
The automotive environment is quite hostile to electronics. Temperature sensors for use on the engine or in the engine compartment must be designed to operate over temperature ranges from -50.degree. C. to +150.degree. C. The supply voltage, although specified at (for example) 5V.+-.0.5V may actually be as high 15.0V and may fluctuate considerably with changing load conditions on the battery and alternator, and with the occurrence of switching transients. Since the RTD temperature is determined by measuring its resistance, the measurement is highly dependent on a voltage or current excitation. In turn, this excitation is dependent on the supply voltage. Accordingly, a need exists for an RTD with a signal conditioning circuit which can provide an analog outlet signal which is functionally related both to RTD temperature and to the supply voltage. Further, the functional relationship must be such as to account for the actual supply voltage at the time a temperature measurement is made.
Preferably, the analog output of the signal conditioning circuitry, when adjusted for supply voltage deviation from nominal, varies linearly with RTD temperature.
To facilitate manufacture, it is also desirable that any adjustments and trimming of the RTD and signal conditioning circuit be performed at one temperature.
In the automotive environment, it is further desirable to provide the RTD and signal conditioning circuitry in a single package, capable of withstanding significant vibration. Certain types of RTD's can be ruled out because they are too sensitive to vibration.
Accordingly, it is an object of the present invention to provide a new signal conditioning circuit for an RTD.
A further object of the invention is to provide for an RTD a signal conditioning circuit, which supplies an output signal which is ratiometric to a supply voltage (i.e., varies linearly with changes in supply voltage).
Still another object of the invention is to provide for an RTD a signal conditioning circuit which is easily calibrated at a single temperature.
A further object of the invention is to provide an RTD signal conditioning circuit suitable for an automotive environment.